We knew that outside groups were playing big in this year’s midterm contests, but the numbers released Tuesday by the Wesleyan Media Project really drove that fact home.

Interest groups have sponsored 59 percent of television ads that have aired in Senate races so far this cycle – up from 51 percent in 2012. The chart below shows the states – outlined in black – where outside groups have run 75 percent or more of the ads that have aired through April 24 on broadcast and national cable.

Not surprisingly, these groups are driving the action in the states with competitive Senate races. In North Carolina, interest groups have run 90 percent of television ads, the Wesleyan study found.

Michigan was not far behind, with outside groups sponsoring almost 87 percent of ads. In Louisiana, independent political organizations have run 85 percent of ads, while in Kentucky, they paid for 75 percent of the spots.

That means a lot of the political debate is being defined by independent players, many of whom give voters little information about their interests or the interests of their donors. More than half of the House and Senate ads run by outside groups were sponsored by organizations that do not reveal their donors ("dark money"), according to an analysis by Wesleyan and the Center for Responsive Politics.

And few of the interest groups have a significant public profile, according to a survey by YouGov commissioned by Wesleyan.

"We’ve seen a huge rise in outside groups that are taking advantage of the relaxed campaign finance atmosphere, and the public knows nothing about them," said Erika Franklin Fowler, co-director of the Wesleyan Media Project. In fact, she added, the public "sees candidates as self-interested, but they give these groups with generic patriotic names the benefit of doubt."

So far, the biggest outside player is Americans for Prosperity, a nonprofit advocacy group that figures prominently in a political network supported by the conservative donors Charles and David Koch. In second place is Senate Majority PAC, a Democratic super PAC that does disclose its backers. But its sister nonprofit, Patriot Majority USA, which does not reveal its contributors, is also in the top five.